Headlights or headlamps are built in to the front of a vehicle, to illuminate the road ahead of the vehicle when driving at night, or in other conditions of reduced visibility.
In headlights two filaments are necessary to provide for a main and a dip beam function. These must be positioned correctly in relation to the highly polished reflector. This is called focusing and is carried out during manufacture.
The main or high beam filament is positioned at the focal point of the reflector to project the maximum amount of light forward and parallel to the reflector axis. This light is then shaped by the lens which is made up of many small glass prisms fused together. These prisms bend the light horizontally and vertically to achieve the desired pattern for road illumination.
The dip or low beam is placed above and slightly to one side of the main filament. Mounting the dip filament in this position produces a beam of light that is projected downwards and towards the kerb side.
With this arrangement the main filament produces the best possible light output while the dip filament gives a downward and dispersed beam which should not dazzle oncoming drivers.
A semi-sealed beam headlight uses a replaceable bulb with a pre-focus collar. The collar locates the bulb in the headlight and also controls the correct positioning of the filaments.
A sealed beam headlight has a highly polished aluminized glass reflector which is then fused to the optically designed lens. This forms a completely sealed unit which has the filaments accurately positioned in relation to the reflector. When a filament fails in a sealed beam light, the whole unit must be replaced.
Some headlight bulbs have a partial shield below the dip filament. This shield stops light from the filament striking the lower part of the reflector. The shield provides the primary shape of the dip beam. The final shaping of the beam is carried out by small cylindrical shaped prisms in the headlight lens. This provides a dipped beam that is asymmetrical.
